There is a considerable (and growing) need in the aerospace and other industries for higher precision and accuracy machine tools than those that are currently being used in production shops. Higher precision and accuracy machine tools enable new manufacturing technologies, such as by eliminating mate-drilling during assembly operations by producing full-size fastener holes during fabrication, etc. Higher precision and accuracy machine tools would also eliminate the need to measure, cut, and fit shims for assembled parts with critical interfaces. The cost for the conventional assembly practices involving measuring and fitting may be substantial—0.5% to 1% of the cost of an aircraft. Thus, the savings related to assembly operations alone of higher precision and accuracy machine tools could save the aerospace industry more than $1 million per aircraft. Additional savings could be realized in the procurement costs for new machines if the higher precision and accuracy could be achieved through software correction, rather than through days, weeks, or months of manual alignment and adjustment of the machine during installation and periodic calibration.
The precision and accuracy of machine tools depends on many factors. A common taxonomy divides machine tool errors into two categories: quasistatic and dynamic. Quasistatic machine tool errors vary slowly with time or the position of the machine. Kinematic machine tool errors arise from the fact that the machine axes are not perfectly square, that the axes are not perfectly straight, that the driving mechanisms are not perfectly made, that the machine was not perfectly assembled, and so on. Load-induced machine tool errors include distortions of the machine due to static or nearly-static forces. An example is the sag of a ram as it extends under the influence of gravity. Thermally-induced geometric machine tool errors (common in most manufacturing environments) include the changes in the dimensions and alignment of the machine due to changes in environmental temperature, and in response to heat sources on the machine, such as the spindle. All of these errors may be measured using traditional equipment, such as laser interferometers, electronic levels, straight edges, and so on. Although the thermal errors are typically slowly changing, it is difficult to arrange a “typical” thermal state of the machine in which to measure the errors, and it is even more difficult to maintain that thermal state during the time required to measure. Dynamic machine tool errors result from the fact that the machine is not moving in a quasistatic way during its normal operation. Rounding or overshoots in corners and following errors as the machine moves on a path involving multiple axes are examples of these errors. These errors are not seen using traditional geometric error measurement techniques because they only appear when the feeds are larger. Spindle error motions are only seen when the spindle is rotating. They include spindle runout and spindle axis precession. Devices are commercially available to measure these errors. Vibrations are periodic motions in the machine, and are, therefore, by definition dynamic. The source of the energy for the motion may be rotating unbalances, the interrupted nature of the cutting operation, and chatter. These errors are completely missed using traditional quasistatic error measurement techniques. Workpiece and tooling errors include chucking and fixturing, tool wear, and material stability.
In attempts to quantify machine tool errors, and to improve the precision and accuracy of manufactured parts, measurements of machine tool operations have been made for decades. In the past few decades, a number of national and international standards have been developed. Most important among these are, in the United States, the ASME B5.54 and B5.57 standards, and, in Europe, the VDI/VDE 3441 and ISO 230 standards. Even in the most modern of these standards, the B5 and ISO series, positioning precision and accuracy are still measured quasistatically. That is, the machine is stepped in linear fashion along each of its linear axes (or in a rotary fashion along each of it rotary axes) and stopped at preset positions. Laser interferometry is used to measure the displacements of the linear axes and either an autocollimator, or a differential angle interferometer combined with an indexing table, are used to measure the displacements of the rotary axes. The dynamic behavior of the machine is typically assessed separately by measuring spindle error motions and contouring accuracy using a telescoping ball bar or its equivalent (for example, a disk check is used in Europe and an encoder that measures positioning in a plane is available).
Clearly, the existing state of the art is inadequate, and becoming more so every day. The existing measurement procedures are very time consuming, such that even making the measurements is expensive. Correction for the quasistatic machine tool errors, while possible (and common in coordinate measuring machines (CMMs)) is rarely performed in a manufacturing environment. Even worse, machining is a dynamic process, and none of the dynamic machine tool errors are measured using current technology. This is further compounded by the changing thermal state of the machine resulting from the varying demand on the spindle and axis drives (i.e. the heat sources in the machine), which are heavily dependent on the part being manufactured. The limitations of the existing measurement techniques are becoming more obvious as the axis speeds and accelerations of machine tools continue to increase. Even for relatively large machines, accelerations are now on the order of 0.5 g to 1 g, and the feed motions are on the order of 40 m/min.
In a yet more demanding requirement, for multi-axis machine tools, the position of the tool with respect to the workpiece must be measured with multiple or all axes in simultaneous movement. Some research has been done into real-time measurement using techniques other than ball bars (which are inherently two-dimensional), but this work has typically been limited to selected straight lines within the workspace because of the limitations of the laser instrumentation. Therefore, at this time, there is a large and growing need for real-time measurement of the performance of multi-axis machines at typical operational feeds and speeds on trajectories that are representative of actual parts. Such measurements, however, are of limited value if they cannot be made quickly to allow for the rapid calibration (and compensation) of the errors discovered. Even more benefit would come from an ability to measure and correct for these errors during machining, providing the ability to take into account the changing thermal and dynamic conditions of the machine.
There are a limited number of three-axis machine tools, and no five-axis machine tools, that have the capability for real-time dynamic calibration and compensation. However, in the past decade or so, there have been technological advances in other fields that may enable new methods and systems for machine tool measurement and compensation. One of the most obvious technological advances is the increase in computational power, combined with increased sampling and data processing capabilities. This opens the door for the real-time measurement of multiple sensors at high data acquisition rates, suitable for data collection at axis speeds consistent with realistic machining operations. Laser measurement technologies have also advanced, allowing greater speed and performance in laser devices, making them suitable as “tracking” devices. Finally, the development of micro-electromechanical systems (MEMS) have led to new classes of very high-performance, relatively low-cost accelerometers, that have low noise and bandwidths down to direct current (DC).
In general, the technology of the present invention takes advantage of the most promising of the above technologies and applies them to the dynamic measurement of multi-axis machine tools. The new hardware and software enables the development of compensation information in a short period of time (i.e. hours, not days, weeks, or months), and the ability to observe the dynamic errors, which the conventional quasistatic methods and systems cannot measure. This research has a direct benefit in defense manufacturing, for example, by dramatically reducing the cost of manufactured parts, through a reduction in set-up and alignment costs for machines, and through an increase in the precision and accuracy of manufactured parts. This is especially true for part families that require large, multi-axis machine tools.
Again, there is a considerable (and growing) need in the aerospace and other industries for higher precision and accuracy machine tools than those that are currently being used in production shops. Higher precision and accuracy machine tools enable new manufacturing technologies, such as by eliminating mate-drilling during assembly operations by producing full-size fastener holes during fabrication, etc. Higher precision and accuracy machine tools would also eliminate the need to measure, cut, and fit shims for assembled parts with critical interfaces. The cost for the conventional assembly practices involving measuring and fitting may be substantial—0.5% to 1% of the cost of an aircraft. Thus, the savings related to assembly operations alone of higher precision and accuracy machine tools could save the aerospace industry more than $1 million per aircraft. Additional savings could be realized in the procurement costs for new machines if the higher precision and accuracy could be achieved through software correction, rather than through days, weeks, or months of manual alignment and adjustment of the machine during installation and periodic calibration. This is an undesirable and costly solution that can directly impact new aircraft delivery schedules because of the long procurement time and high costs for new machines. Additionally, the process must be frequently repeated to ensure that the machine calibration remains correct.
Volumetric calibration and compensation is sometimes seen as a solution to reduce the build-time for machines, and to improve the positional accuracy for the production of high precision and accuracy components. However, full volumetric calibration and compensation of multi-axis machines (i.e. four or more axes, with at least one rotary axis, for example) is still not common among aerospace suppliers or the like. The main reasons for this are that current methods of calibration and compensation do not identify all of the geometric and motion errors of an arbitrary machine, many of which significantly impact the volumetric accuracy of the machine—in particular, the non-quasistatic errors are missed; they require expert setup and the use of multiple metrology systems; and they may require a week or more of non-productive machine down time.
The result is that many aerospace suppliers and the like simply do not calibrate their machines. The demand for reliable high precision and accuracy machine tool capability is passed to the machine builder, who must then spend weeks (and sometimes months) trying to build precision and accuracy in, instead of rapidly deploying the machine and then calibrating it. The associated costs appear in the final products of the machines. If instead the capability of volumetric calibration and compensation of machine tools may be improved to provide a faster and more complete error measurement, and to correct for those measured errors, it would open the door for significant cost savings.